Mechanical compression or injury to spinal nerves with resulting radicular pain can develop in response to a variety of conditions, including spondylolisthesis, osteoarthritis, and degenerative disc disease, among others. Nerve root irritation can also result in numerous symptoms aside from the radicular pain, including both sensory and motor deficiencies, such as numbness of the extremities, weakness, and difficulty with or loss of dexterity and muscle control.
One method for controlling pain resulting from irritation of a nerve root is electrical stimulation of the dorsal nerve rootlet and associated dorsal root ganglion, effected by an electrode array implanted peripheral to the dorsal rootlet. The dorsal rootlet transmits sensory signals, and stimulation of the dorsal rootlet can alleviate painful sensations without interfering with motor functions, which are transmitted through the adjacent ventral rootlet.
The dorsal root ganglion is located along the dorsal rootlet and contains the cell bodies of the neurons whose axons traverse the dorsal root. Stimulation of the dorsal root ganglion is a promising treatment for neuropathic pain. Research shows that over half of patients with chronic back pain see a reduction in symptoms with nerve stimulation, or neuromodulation, procedures.
To maximize the efficacy of dorsal root stimulation, implantation of the electrode array should be proximate to the dorsal root ganglion. Location at greater distances requires increased amounts of energy to be delivered to the electrode array to achieve the desired stimulation, which depletes energy sources more quickly. Prior art solutions, such as implantation of an electrode array on the exterior of the vertebrae running parallel to the spinal column, suffer from this problem. Furthermore, location outside the spinal vertebrae leaves the electrical stimulation signal subject to dissipation due to bulk conductivity of the surrounding soft tissues and cerebrospinal fluid.
The technical skill required to properly locate the electrode adjacent the dorsal root ganglia presents a challenge because of the anatomical orientation of the nerve rootlet and dorsal root ganglion within the intervertebral foramen. Additionally, the site of the ganglia varies depending on the location of the vertebrae of a single patient. For example, one survey found the location of the dorsal root ganglia in the fourth lumbar spine to be intraspinal (“IS”) in approximately 6% of patients, intraforminal (“IF”) in approximately 48% of patients, and extraforminal (“EF”) in 41% (5% were not identified). In the fifth lumbar spine, the same population had 10% IS, 75% IF, and 6% IS (with 9% not identified) dorsal root ganglia.
Prior art techniques address this problem by percutaneously injecting the electrode through the intervertebral foramen via a needle, laying it alongside the spinal nerve root. However, this technique leaves the electrode suspended in the intervertebral foramen without firm fixation. Hence, the electrode array is prone to migration, which both diminishes the efficacy of the stimulation technique and can cause other complications necessitating surgical correction of the migration or removal of the electrode array.
Percutaneous injection is also not an ideal solution because it carries with it all the risks, costs, and time constraints normally associated with surgery. Further, surgery may not be an option for certain patients because of risk factors such as age, clotting, prior injuries, and pre-existing epidural scars.
The problem of electrode array migration has been addressed by other prior art techniques, but has not been adequately resolved. For example, prior art techniques to anchor the electrode include an anchoring hook that is engaged in the fibrous fascia layer surrounding the nerve root. The anchoring hook must pierce the nerve fascia layer. But, piercing the nerve fascia layer with a hook presents risk of nerve damage. Migration also remains a problem with this technique.
U.S. Patent Publication No. 2017/0021180 to Datta discloses a method for implantation of a neural stimulator comprised of electrodes attached to a generator. The electrodes are connected to the generator via a subcutaneous lead with connector plugs. However, the method anchors the electrode to the soft tissue near the targeted nerve, which leaves the electrode susceptible to migration.
U.S. Patent Publication No. 2016/0199112 to Kim discloses a medical insertion apparatus comprised of a screw nail body to be implanted in a boney structure that includes an electrode. The screw nail body includes an electrode connected to a lead which runs along the length of the screw nail body either inside a cavity or along the outside edge, or a combination thereof. The position of the electrode is fixed at the terminal end of the screw nail body, requiring the screw nail body to be located immediately peripheral to the targeted nerve, which is not always possible when targeting the dorsal root ganglion. Furthermore, the screw nail body must be seated perpendicularly to the surrounding bone, prohibiting an electrode position parallel to the nerve root. Alternatively, using an array of electrodes that extends beyond the tip of the screw nail body leaves the array adrift in the epidural space, with no way to position the array precisely and no way to control electrode migration.
U.S. Pat. No. 6,356,792 to Errico, et al. discloses an assembly for securing an electrode inside a patient's skull. A skull port member is affixed to the skull. An electrode is placed inside the skull and the connecting lead is run through the skull port member. The electrode is secured by a mechanism that seats in the skull port member and crimps the connecting lead. However, the electrode is susceptible to movement when the operator inserts the lead-locking mechanism into the skull port member and crimps the connecting lead. The nature of the mechanism also limits the possible materials and possible sizes of the assembly, as thinner and lighter materials in the connecting lead would be likely to break when crimped in place by the lead locking mechanism. Furthermore, the design is ill-suited for use in the spine, as there is no way to position the electrode perpendicular to the direction of the skull port member, which is desirable for stimulation of spinal nerves.
U.S. Pat. No. 9,737,233 to Londot discloses an assembly having a pedicle screw with an electrically-conductive longitudinal member that is used to propagate a signal along the exterior of the pedicle screw. However, the assembly does not allow for placement of the electrode beyond the pedicle screw and limits locations to which electrical stimulation can be applied.
U.S. Pat. No. 9,579,222 to Branemark, et al. discloses a percutaneous gateway for transmission of signals from a patient's nervous system to a robotic prosthesis. The system discloses an apparatus for mounting a prosthesis and preserving the percutaneous transmission of signals with appropriate seals to prevent infection after long-term use, as well as use with stimulating electrodes that may optionally be implanted. However, the system does not disclose a method for locating the electrodes relative to targeted nerves, anchoring the position of the electrodes, or implantation in the spine.
Hence, there remains a need for an electrode array and implantation technique that can reliably locate the array within close proximity to the dorsal root ganglion, regardless of the ganglion site, and effectively anchor the array in place to reduce or eliminate future migration.